1. Field of the Invention
This invention relates to ultrasonic imaging systems and, more particularly, to an ultrasonic imaging system that functions effectively in both two dimensional (2-D) and doppler modes.
2. Description of the Related Art
Duplex ultrasonic imaging systems operate in both two dimensional (2-D) and doppler modes. These duplex systems, however, have suffered from poor performance when operated in the doppler mode. For example, a phased array ultrasound system might utilize only eight-bit resolution parallel (flash) analog to digital (A/D) converters for two-dimensional (2-D) imaging. Higher resolutions, however, are required for the ultrasonic imaging system to operate effectively in the doppler mode.
One way to achieve higher resolution is to extend the bits processed by the flash A/D converter, but the cost and complexity of the circuit are greatly increased as a result. The reason for this increased cost and complexity is that a flash converter simultaneously compares an input analog voltage to 2.sup.n -1 threshold voltages to produce an n-bit digital code representing the analog voltage. An 8-bit flash A/D converter thus requires 2.sup.8 -1 comparators, or 255 comparators. Therefore, extending the flash A/D converter to increase the resolution above that required by the 2-D mode is costly due to the exponentially increasing number of analog comparators required for an ultrasound imaging system. For example, 10-bit resolution would require 1,023 comparators, and 12-bit resolution would require 4,095 comparators.
In light of the foregoing, there exists a need for an ultrasonic imaging system that can operate effectively in both the 2-D and doppler modes, without incurring the added cost and complexity of higher resolution flash A/D converters.